Editing the normal direction to be parallel with the face.
If you want to modify specific normals of specific faces of a mesh, the only way I know of is the "Normal Edit" modifier.

Basically you create a vertex group, containing all the faces you want the normals changed of. Then add the Normal Edit modifier, choose Directional, select the vertex group and pick a target object. After that all normals of all faces of the selected vertex group will point in the direction of the center of the target object. If you want the normals be parallel, select Parallel Normals in the modifier.